PRAGUE (AFP) - US President George W. Bush on Tuesday accused Russia of derailing democratic reforms, in a speech likely to widen a bitter rift with Moscow on the eve of the G8 leaders summit in Germany.

Bush's criticism came even as the president sought to soothe Russian fury over US plans to extend a missile defence shield in Central Europe -- a move that Russian President Vladimir Putin has warned could reignite an arms race.

The escalating dispute between the old Cold War foes is threatening to overshadow the gathering of leaders from the Group of Eight (G8) world powers in Germany on Wednesday and distract from the main agenda of combatting climate change and alleviating poverty in Africa.

Bush said he would assure Putin that "Russia is not our enemy," and that the proposed extension of the anti-missile umbrella into the Czech Republic and Poland was a purely defensive measure "aimed not at Russia but at true threats."

"The Cold War is over. It ended," Bush told reporters during a brief stopover in Prague en route to the G8 summit in the northern German seaside resort of Heiligendamm.

Addressing a joint press conference with Czech President Vaclav Klaus and Premier Mirek Topolanek, the president even opened the door for any skeptical Russian military brass to inspect the system for themselves.

"Why not send your generals over to see how such a system would work. Send your scientists, let us have the ability to discuss this issue in an open forum," he said.

Russia has refused to accept repeated US assurances that the shield is aimed at defending against an attack from the likes of Iran, which denies Western suspicions that it is striving to create a nuclear arsenal.

Putin has warned that Russia will target Europe with its own missiles if the shield deployment goes ahead.

Later Tuesday, in an address to a democracy conference in Prague, Bush risked further aggravating Putin by questioning his government's democratic credentials. Washington has previously criticised the Russian leader for his authoritarian leadership style.

"In Russia, reforms that once promised to empower citizens have been derailed, with troubling implications for democratic development," Bush said.

His remarks were couched in references to the "complex" US friendship with Russia, which he compared to Washington's often difficult relationship with China.

"Part of a good relationship is the ability to talk openly about our disagreements. So the United States will continue to build our relationships with these countries -- we will do it without abandoning our principles or our values," he stressed.

As Cold War-era relations with both South Korea and Taiwan proved in the past, "America can maintain a friendship and push a nation toward democracy at the same time," he added.

His remarks are likely to further rile Putin, who recently condemned US "imperialism" in world affairs and warned that the US missile defense plan would turn Europe into a "powder keg."

Designed as a defence against ballistic, or intercontinental, missiles that can fly more than 5,000 kilometres (3,100 miles), the shield includes a ground component of eight missile interceptors in Alaska and two others in California.

To complete the system, Washington announced in January that it wanted to install 10 interceptors in Poland and a radar system in the Czech Republic, which would be linked to an early warning radar, probably in the Caucasus.

During his brief stay in Prague, Bush held talks on the proposed deployment with Klaus and Topolanek, both of whom have stated their backing for the plan.